Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

while the fraction of opportunists increased over the sampling years. The
species number was similar in 2001 and 2003 and slightly lower in 2005.
Seasonal variations were also significant for all variables, except for the
cumulated cover of late-successionals and the fraction of opportunists
(Table 3.5).
Dependence on substratum composition
Modelling of algal variables was improved by taking into account that
algal cover varied with the level of hard substratum (Table 3.5). For details
see Carstensen et al. (2005).
Stochastic variation in algal cover
In addition to the fixed variation due to area, depth, substratum composition
in depth intervals and temporal variation, the algal variables are
also subject to stochastic variation. The stochastic variation has been
subdivided into variation between sites, variation due to diver effects
and residual variation (Table 3.6). The residual variation expresses the
variation between replicates, i.e. the random variation which is left when
the other stochastic effects have been taken into account. In case the
model is not perfect, the residual variation also includes variation due to
imperfection of the model.
For 'total cover', the residual variance is 0.0674. This equals a standard
error of 0.26 (= sqrt 0.0674) of arc. sin. transformed total cover levels or
an absolute variance around 16.5% (calculated by up-scaling with Π/2,
valid for intermediate coverages). This means that an algal cover value of
50% could typically be represented by observations between 33.5 and
66.5%. If more sites are included in the survey, site-to-site variance,
which is slightly smaller than the residual variation, adds to the variability.
Moreover, divers also contribution to random variation, that should
be included. Total stochastic variance is 0.1556 which recalculated to total
cover values corresponds to about 25% at cover levels of 50%. Thus, a
mean total cover of 50% could typically be represented by observations
between 25 and 75%. The variance is smaller at the extremes of the scale.
Cumulated cover is even more variable. The residual variation of 0.2981
corresponds to a standard error (se) of 0.55 (= sqrt 0.2981) of the log
transformed cumulated cover value. This equals an absolute variation of
73% (calculated as exp(se) = 1 + d, where d is the relative variation). If
more sites and different divers are involved, the total variance amounts
to 0.7613, corresponding to a variation of about 139%. Thus, a mean cumulated
cover of 200% might typically be represented by observations in
the range 143%-278%.
Stochastic variation of cumulated cover of perennials or opportunistic
species is even larger, and those of the fraction of opportunists and the
number of late-successional algal species are also large. So, in conclusion,
all the algal variables are characterised by large variability.
Gradients within sites
We chose 11 areas having a sufficient number of sites to enable the identification
of potential spatial gradients. Augustenborg Fjord, Flensborg
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